It depends on the conditions that the problem gives. For example, if the question states that there is infinitesimal changes, use the reversible equation. When the change is too large(cannot by reversed by an infinitesimal change), then use the irreversible equation.

Only the problem can tell exactly which type of reaction it is without calculating delta G (Gibbs free energy). If there are many many changes (as stated by Chloe) then reversible is the way to go, however if it is sudden and drops quickly then it is irreversible.

Usually it'll be given but the biggest difference has to do with the amount of pressure and if it changes "infinitesimal" Its a reversible process if the pressure is being slightly changed like 1 millionth less compared to the pressure of the system. If the pressure change is a large number then it's irreversible.